Abstract
A turbulent jet with a Reynolds number of 71,000, controlled by a pair of pulsed jets operating 180° out of phase, is investigated by large-eddy simulation. The effect of the forcing amplitude on jet development and generated coherent structures is investigated through six cases: an unforced case and five cases at different amplitudes. With this forcing mode, the jet bifurcates in the far field that promotes the spread of the shear layer. It was found that the jet yields the largest bifurcation angle at medium amplitude, while the bifurcation is restrained at lower or higher amplitudes. As the amplitude increases, the streamwise vortex pair generated by the control jet is stronger and penetrates deeper into primary jet. This phenomenon reduces the inclination of vortex ring when mutual interaction between vortex ring and streamwise vortex pair occurs. Meanwhile, at higher amplitudes, the inclined vortex ring is stronger and unevenly distributed, prompting the jet bifurcation. In order to investigate the coupling effect of amplitude and frequency, different forcing frequencies were also simulated. The results demonstrated that the optimal frequency based on centerline velocity decay rate decreases with increasing amplitude; however, it eventually saturates around StD = 0.1.
Highlights
In recent years, flow control techniques for round jets have received wide attention.This is primarily due to their potential and capabilities for performance improvement in engineering applications, in the aerospace field
When an active control system has been properly configured, it can provide comparable or even better performance and less flow loss compared to passive configurations [3]
The reason is that pulsed jets can penetrate the primary jet, but can introduce periodic perturbation, which responds to the primary instabilities in the jet
Summary
Flow control techniques for round jets have received wide attention. Tay [12] used two steady control jets to investigate the effect of mass flow ratio on round jet structures Their results revealed that the presence of secondary injection inhibited the largescale vortex roll-ups and made the primary jet shear layer transit to turbulence earlier with increasing injection flow rate. As the mass flow ratio increased, the CVPs penetrated deeper into the primary jet until they eventually met In their quantitative analysis, the centerline velocity decayed faster at higher injection rates, indicating improved mixing. Da Silva et al [19] investigated the flapping mode excitation at preferred mode and the subharmonic frequencies Both forcing methods succeeded in causing the jet bifurcation at low Reynolds numbers. Some additional cases at different frequencies are simulated to investigate the coupling effect of forcing amplitude and frequency
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